Infusion packages for brewing beverages, such as tea bags and coffee bags, are generally produced from either “heat seal” or “non-heat seal” fibrous nonwoven web material. Heat seal webs generally comprise two layers or phases. One of these two layers includes fusible polymeric fibers which allow the web to be heat sealed to itself in the production of infusion bags. The other layer is present as an insulation layer to prevent polymer sticking to heated dies during conversion of the web to produce an infusion package. In contrast, a non-heat seal paper, which normally has a basis weight in the range of 9 to 18 g/m2, and typically about 12.3 g/m2 is generally comprised of a single layer comprised of vegetable fibers and does not incorporate fusible polymeric fibers. Thus, as its name suggests, non-heat seal webs cannot be heat sealed to themselves. Infusion bags are produced from such material by crimping or otherwise mechanically securing two portions of the material together.
There is however a problem is some areas with conventional non-heat seal materials for use in the production of tea bags in that fine particles of tea or tea dust resulting from interaction of tea leaves during processing have a tendency to pass or sift through the web material to the outside of the tea bag. Since tea bags are generally packaged in boxes or other types of “outer” packaging, the fine tea particles or dust are “loose” in the packaging and this is undesirable from an aesthetic viewpoint.
One possibility of overcoming this problem would be to increase the percentage of finer fibers, such as hardwood fibers, in the stock from which the single ply nonwoven web material is produced. This would result in a nonwoven with smaller pores, thus reducing the amount of fine tea or tea dust which can sift through the web. The increase of hardwood fibers or other short fibers in the single layer to achieve the required pore size distribution would, however, affect overall strength to the extent that the web would not have sufficient strength for manufacture into infusion bags. A further disadvantage which would be associated with the use of hardwood fibers would be the incidence of pin-hole generation through air entrainment.
A further disadvantage of conventional non-heat seal material is that it is difficult to provide a pattern in the web using conventional methods. The patterns which are desired are those which can readily be produced in nonwovens of the “heat seal” type. Such patterns may comprise a matrix of perforations or semi-perforations which are formed through the web and which are intended to allow the passage of water therethrough. Alternatively, the pattern may be either a logo or other marking indicating the manufacture of either the nonwoven or the infusion packets prepared therefrom.
Such perforations are generally formed in heat seal nonwovens by one of two methods. Firstly, the perforations may be formed by a pattern of projections known as knuckles on the forming wire of the papermaking machine on which the fibrous suspension used for producing the paper is produced during the “wet laying” operation. Secondly, the pattern may be formed in the web by fluid jets directed onto the formed web.
Non-heat seal nonwoven webs are generally comprised of a single layer and typically have a basis weight of 12.3 g/m2. The patterning methods discussed above is not generally used for such non-heat seal webs. Thus, if the web is formed on a wire provided with knuckles, it cannot be easily released from the wire. This is believed to be due to the fact that the cellulosic fibers of the web are more cohesive because of their greater contact with the wire and their wetness, and as such are more difficult to release than the cylindrical synthetic fibers used in heat seal nonwoven materials. If a liquid jet is used to pattern such a non-heat seal web, the resultant material is too “open” as the jet tends to “strike through” the single layer and allow beverage precursor material, such as tea leaves, to pass or sift through the web material.